Merge libcxxrt commit 45ca8b1942090226ba9368caeeeabc0d4ee41ad6

Insert padding in __cxa_exception struct for compatibility

  Similar to https://github.com/llvm/llvm-project/commit/f2a436058fcb, the
  addition of __attribute__((__aligned__)) to _Unwind_Exception (in commit
  b9616964) causes implicit padding to be inserted before the unwindHeader
  field in __cxa_exception.

  Applications attempt to get at the earlier fields in __cxa_exception, so
  preserve the same negative offsets in __cxa_exception, by moving the
  padding to the beginning of the struct.

  The assumption here is that if the ABI is not aware of the padding
  before unwindHeader and put the referenceCount/primaryException in
  there, no padding should exist before unwindHeader.

This should make libreoffice's custom exception handling mechanisms work
correctly, even if it was built against an older cxxabi.h/unwind.h pair.

PR:		263370
MFC after:	3 days

contrib/libcxxrt/cxxabi.h

@@ -77,6 +77,13 @@ typedef void (*terminate_handler)();
 struct __cxa_exception
 {
 #if __LP64__
+	/**
+	 * Now _Unwind_Exception is marked with __attribute__((aligned)), which
+	 * implies __cxa_exception is also aligned.  Insert padding in the
+	 * beginning of the struct, rather than before unwindHeader.
+	 */
+	void *reserve;
+
 	/**
 	 * Reference count.  Used to support the C++11 exception_ptr class.  This
 	 * is prepended to the structure in 64-bit mode and squeezed in to the